1. Field of the Invention
The present invention relates to an automatic constant exposure system including a camera speed aperture computer and an iris-ring drive mechanism for use with a motion picture camera and more particularly to an automatic constant exposure system which provides constant film exposure independent of varying camera speed.
2. Description of the Prior Art
In his article, entitled "A New Automatic Iris Control for Motion-Picture Cameras", published in the Journal of the SMPTE, Volume 66, July 1957, pages 413-416, Mervin W. La Rue, Jr. has discussed a motion picture camera which incorporates a self-powered compact iris control which takes into account camera speed. The relationship between iris-ring rotation and the resultant iris-opening area is essentially a logarithmic function in most lenses. More specifically, a change in exposure of one f-stop requires the same amount of angular iris-ring movement throughout the entire range of the iris. The system is based on the well-known "exposure meter equation" which is f.sup.2 =KB.sub.0 ST where f is the lens-iris setting, K is a constant, B.sub.0 is the average subject brightness in candles per square foot, S is the "ASA" emulsion index of the film used and T is the exposure time in second with the logarithm of both sides of the "exposure meter equation" being: log f.sup.2 =log K+log B.sub.0 +log S+log T. Since the lens-iris is "linear," its angular deflection, I, is proportional to log f.sup.2 and the "exposure meter equation" now becomes: I=C log f.sup.2 =C (log K+log B.sub.0 +log S+log T). The desired total meter deflection, M, is the iris deflection multiplied by the gear ratio, G, between the contact gear and the iris ring and the "exposure meter equation" finally becomes: M=CG log f.sup.2 =CG (log K+log B.sub.0 +log S+log T). Each of these terms has a particular significance. The term CG log T is related to the exposure time and is inversely related to the camera speed. In the motion picture camera which La Rue has described in his article the camera operator manually sets the control barrel for the camera speed to be used. The camera operator does not have the ability to vary the speed of the camera during the shooting of a scene and therefore he does not need to change this setting of the control barrel.
U.S. Pat. No. 4,142,785, entitled Exposure Control Circuit for Camera, issued to Seiichi Saito on Mar. 6, 1979, teaches an automatic circuit for intermittently sensing adequate light condition and adjusting a cine camera iris for same. The circuit includes an astable multivibrator for intermittent switching of power to a Cds cell. Under low light condition the resistance of the cell becomes great enough to stop the oscillation of the multi-vibrator. Prior to the invention described in U.S. Pat. No. 4,142,785 a galvanometer type motor, which was fragile and therefore subject to damage caused by rough handling, automatically closed the iris in accordance with incoming light. In U.S. Pat. No. 4,142,785 the use of a servo motor effectively circumvented the reliability problems but added a new problem in that excessive power consumption was required if the system was permitted to stay on when the camera was not being used to feed film for shooting.
U.S. Pat. No. 4,213,683, entitled Electronic Shutter Driving Device for Camera, issued to Masamichi Furukawa on July 22, 1980, teaches an electronic shutter driving device having a logarithmic expansion circuit which includes no temperature-dependent semiconductor elements such as transistors and field effect transistors and which includes an operational amplifier, a series of resistors which are connected to an inverting input terminal of the operational amplifier, a plurality of capacitors which are interconnected between the inverting input terminal of the operational amplifier and the junctions, respectively, between the resistors and an integrating capacitor between the inverting input terminal and connected between the inverting input terminal and the output terminal of the operational amplifier. A source voltage is applied to the output terminal or a reference voltage input terminal of the operational amplifier. Prior to the invention which U.S. Pat. No. 4,213,683, teaches the electronic shutter driving devices include a logarithmic compression circuit and a logarithmic expansion circuit. The logarithmic expansion circuit generally consisted of transistors and field effect transistors and its operation varied widely depending on the temperature.
In a Moviecam brochure there is a description of a control unit which attaches to a motion picture camera and which allows running speeds to be changed at programmed rates during the shooting of a scene. The time interval in which acceleration or deceleration takes place can be preset on switch banks. Speed changes are made with full automatic exposure corrections via the automatic iris control servo motor coupled to the lens iris ring. The processor is a digital device wherein the iris-ring drive mechanism moves the lens iris-ring in discrete steps rather than in a continuous movement. This is a problem because rapid changes in camera speed may cause non-uniformity in film exposure which a viewer perceives when these changes occur very rapidly. The processor is also dedicated for use only in the Moviecam motion picture camera. The control unit can not be used with other motion picture cameras. Furthermore there are no means provided to interface the control unit with lenses other than those lenses with a particular aperture spacing between f/stops.
In their article, entitled "Special Effects for FIREFOX", published in American Cinematographer, Volume 63, Number 9, September 1982, which begins on page 912, John Dykstra, Roger Dorney, Grant McCune, John Sullivan, Mat Beck and Don Trumbull discuss a variety of special effects which they used in the filming of FIREFOX. Among these special effects was the use of electronics which compared and balanced camera speed versus f/stop and which allowed the filming to be done at camera speed which were changing from 1/2 of a frame per second to 24 frames per second without any visible fluctuation in the final result. The motion picture camera incorporated an exposure compensation device which included a synchro motor which is mechanically coupled to the lens iris by a backlash free gearing. The electronics included a personal computer and a device for controlling camera frame and for counting frames which is electrically coupled to the computer and which signals the computer when it has counted a frame. The electronics also includes a device for controlling the lens iris through a remote servo motor and, when given a reference exposure, computing and setting the proper f/stop for every camera speed from 24 frames per second to 1/2 per second so that in the filming of the jet in FIREFOX the apparent speed of the jet could be varied continuously with constant exposure and proper streaking. The difficulty with using the personal computer for the electronics is that the personal computer is bulky in size which is disadvantageous and is also inefficient because its hardware and software are not dedicated to the purpose of maintaining constant film exposure in response to changes in camera speed of a motion picture camera.